This invention refers to an innovative method of control for a numerical control machining apparatus, for example of the type with three cartesian axes described in the Italian patent application MI92A000491 on behalf of the same Applicant.
The problems caused by sudden sharp changes of direction in automatic precision control mechanisms, such as numerical control machining apparatuses, are well known. Inertia of the masses in movement prevents excessively sharp changes due both to the considerable stress transmitted to the mechanical parts and to the possible loss of control of the movement.
For example, as is well-known to the expert, stepper motors are often advantageously used in similar applications, thanks to the low cost of the necessary electronic control equipment and to its intrinsic characteristics. Conversely, these motors cannot be subjected to sharp changes in the speed of rotation, or of the control pulse period. In fact, an excessively rapid change gives rise to an inevitable loss of pitch, that is to say, the angular position assumed by the motor becomes indistinct, with obvious consequences as regards the accuracy of the controlled movement.
Especially in cartesian-axis machining apparatuses, such as the one described in the aforementioned application, each change of direction of the tool is a potential source of problems, due to the fact that each movement in the cartesian space is made up of a combination of movements along the three cartesian axes, each of which is controlled by its respective motor.
Sharp-cornered points along the path of movement therefore give rise to sharp changes in the speed ratios between the three motors. The problem is usually avoided by slowing down the motors along pre-established ramps, in the vicinity of each change of path of the tool, for example until they come to a standstill, and then gradually re-accelerate to the normal machining speed after having passed the point of change of direction, thereby ensuring that the changes in speed of the individual motors remain within a given safety limit.
This method of proceeding however results in a slow down of the production process, since the average speed of movement of the tool depends upon the number of changes in direction. Moreover, it has been observed that in machining operations on materials such as wood, the speed of movement of the tool is an extremely important parameter for the surface finish of the area subjected to the action of the tool itself. Consequently it is preferable to keep speed variations along the path to a minimum.